Modern automotive original lacquer coatings generally consist of a base lacquer/clear lacquer topcoat, which is applied onto a body which has been electrophoretically primed and coated with surfacer. In such coating processes, the coloured and/or effect base lacquer and the clear lacquer are preferably applied wet-on-wet, i.e. the base lacquer and subsequently applied clear lacquer are jointly stoved.
The optical/aesthetic qualities of such base lacquer/clear lacquer two layer lacquer coatings are substantially influenced by the quality of the clear lacquer layer. ideally, the texture of the clear lacquer layer is identical on the horizontal and vertical surfaces of a three-dimensional substrate, for example an automotive body, which may be ascertained, for example, by determining the long- and short-wave components of the surface texture of the clear lacquer surface. This ideal situation is, however, not straightforward to achieve in practice. For example, differences in surface texture or leveling of the outer clear lacquer layer may occur as, once applied and in particular during the heating phase of the stoving process, the clear lacquers have a tendency to run or sag on non-horizontal surfaces due to a temperature-determined reduction in viscosity. There are various ways of counteracting unwanted running phenomena in clear lacquers. For example, additives having a favourable effect on running behaviour are added to the stoving clear lacquers.
However, this frequently results in reduced gloss of the clear lacquer layer, if the clear lacquer layer is compared with a clear lacquer layer produced from a corresponding, additive-free stoving clear lacquer. Another possibility for counteracting unwanted running phenomena in clear lacquers involves rotating the substrate around an axis after application of the stoving lacquer during the flashing-off and/or stoving operations, an approach which is disclosed, for example, in EP-B-0 261 644.
Longer term retention of the optical/aesthetic qualities of base lacquer/clear lacquer two coat lacquer coatings, in particular in automotive applications, is substantially determined by the clear lacquer layer's resistance to chemicals and scratching, which is in particular manifested over a relatively long period of service by a greater or lesser resistance to acid rain and washing scratches.
EP-A-0 000 407 describes a radiation-curing coating composition based on a hydroxy-functional polyester resin esterified with acrylic acid, a vinyl compound, a photoinitiator and a polyisocyanate. In a first curing stage, the coating is radiation-cured by UV light and in a second curing stage hydroxyl/isocyanate crosslinking imparts the final hardness to the coating composition. The second curing stage may proceed at elevated temperature.
EP-A-0 247 563 describes a radiation-curable coating composition based on a poly(meth)acryloyl-functional compound, a polyol mono(meth)acrylate, a polyisocyanate, a light stabiliser and a photoinitiator. Radiation curing proceeds by UV light. The hydroxyl functions in one part of the radiation-curing binder provide an additional possibility for curing the polyisocyanate, for example in shaded areas.
EP-A-0 540 884 describes a process for the production of a multilayer lacquer coating by applying a clear lacquer coating onto a dried or crosslinked coloured and/or effect base lacquer layer, wherein the clear lacquer coating is applied from a coating composition which is curable solely by free-radical and/or cationic polymerisation and curing is initiated and/or performed by high-energy radiation.